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History
It is not known clearly when people first began to use glasses.
They probably began to use glasses in the thirteenth century. Marco
Polo is said to have mentioned that he saw the Chinese wearing
glasses in the year 1275.
The need and demand for
glasses increased with the advent of printed books in the late
fifteenth century. Glasses for farsightedness began to be used in
the sixteenth century. Bifocals serving both purposes (one half of
the lens enabling far vision and another half making near vision
easy) were introduced in 1784 by Benjamin Franklin, the American
scientist.
Corrective glasses
Corrective glasses are prescribed for
focusing problems. These include - near sightedness (short sight or
myopia), far sightedness (long sight or hypermetropia), astigmatism,
presbyopia (a condition that sets in usually when a person is in his
forties, he gradually loses the ability to focus on near objects)
and strabismus (cross-eye).
What glasses are
prescribed
For myopia, concave lenses that are thinner in the middle and
thicker at the edges are prescribed. For hypermetropia, convex
lenses, those that are thicker at the centre and thinner at the
edges are advised. In the case of presbyopia, the patients are
prescribed convex lenses for close vision and reading. Since many
strabismus results from farsightedness, children with the condition
are prescribed glasses to correct far vision. If glasses are not
used, one eye may become weaker and weaker over the years.
As mentioned earlier,
glasses are prescribed after a thorough examination of the patient.
Normally, when light enters the eye, it gets refracted (bent).
During examination, the ophthalmologist or optometrist measures how
much more or less each eye of the person needs to refract light rays
so as to focus on the retina. For this, the person is made to look
through different lenses and then the professional decides on a
reading that will offer him best vision with the corrective glasses.
An optician makes them
according to prescription and shapes them to fit into a frame. It is
also the optician's job to see the frame sits on the person
comfortably, offering him the best possible vision. The lenses are
made from glass or plastic. Safety glasses made for use by
industrial workers and for other specialized purposes are made from
extremely strong glass or plastic. |
